Rollup merge of #112867 - compiler-errors:more-impl-source-nits, r=lcnr

More `ImplSource` nits

Even more clean-ups, I'll put this up in parallel with the `select_in_new_trait_solver` PR.

r? ``@lcnr``

compiler/rustc_trait_selection/src/traits/project.rs

@@ -1720,7 +1720,6 @@ fn assemble_candidates_from_impls<'cx, 'tcx>(
         };
 
         let eligible = match &impl_source {
-            super::ImplSource::TraitAlias(_) => true,
             super::ImplSource::UserDefined(impl_data) => {
                 // We have to be careful when projecting out of an
                 // impl because of specialization. If we are not in
@@ -2012,8 +2011,7 @@ fn confirm_select_candidate<'cx, 'tcx>(
         }
         super::ImplSource::Object(_)
         | super::ImplSource::Param(..)
-        | super::ImplSource::TraitUpcasting(_)
-        | super::ImplSource::TraitAlias(..) => {
+        | super::ImplSource::TraitUpcasting(_) => {
             // we don't create Select candidates with this kind of resolution
             span_bug!(
                 obligation.cause.span,